NK cells provide a first line of defense against a variety of microbial agents and are potent producers of cytokines that modulate the response of many other cell types. In addition, experimental and clinical studies highlight the potential for NK cells to enhance anti-tumor responses. However, relatively little is known about the combination of positive and negative signaling events that regulate these processes. In fact, recent published work and preliminary data presented in this application suggest that the current paradigm for T cell activation is insufficient to explain signal transduction in NK cells. As an alternative model, the applicant hypothesizes that (a) unlike T cells, NK cells preferentially utilize the protein tyrosine kinase Syk during their activation and this enables them to become activated without the participation of Src-family kinases; (b) RhoA, Cdc42Hs, and Rac1, three members of the Rho family of G proteins, separately influence distinct steps contributing to the development of NK cell-mediated cytotoxicity; and (c) MHC-recognizing receptors on NK cells inhibit NK cell activation by directly interrupting Syk-dependent signaling. In order to test these hypotheses, the applicant will: (1) Evaluate the differential roles of Syk-family (i.e., ZAP-70 vs Syk) tyrosine kinases in NK cell activation; (2) Identify and characterize the aspects of NK cell activation regulated by the Rho-family of low molecular weight G proteins; and (3) Determine the mechanisms regulating the inhibition of NK cell activation by MHC-recognizing killer cell inhibitory receptors. Together these studies will provide an experimental basis for understanding the molecular events that are involved in the regulation of NK cell effector functions, and will, in a broader context, advance our understanding of the fundamental processes involved in transmembrane signaling and lymphocyte activation.